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Bundle represents a directory. It groups resources together.

Application

In a typical application, all of your assets, images, strings, resources and code will be in the “main bundle”.

To be convenient, resource classes provide sensible default to use.

// NSLocalizedString function signature has a default bundle
NSLocalizedString(_ key: String, tableName: String? = default, bundle: Bundle = default, value: String = default, comment: String) -> String

The default is Bundle.main. So you don’t have to specify explicitly.

Without surprise, UIImage initialization uses the main bundle too. And if you want to specific another bundle you could use the other init.

But the default to use main bundle is not applicable to frameworks and playgrounds.

Framework

When you are developing a framework, you should NOT use Bundle.main, because that is the app’s bundle.

Your resources is in your framework’s bundle.

Therefore the default main bundle that NSLocalizedString and UIImage use will fail in when you are developing a framework.

The correct way:

let frameworkBundle = Bundle(for: MyFrameworkClass.self)
let stringInFramework = NSLocalizedString(key, bundle: frameworkBundle, comment: "")
let imageInFramework = UIImage(named: "img", in: frameworkBundle, compatibleWith: nil)

Bundle(for:) conveniently find the bundle that contains the class. You could use any other class in your framework to replace MyFrameworkClass. It works because all code and resources will be in the same bundle for a framework.

Playground

Playground is another special case.

Try running this in playground to see the bundlePath (the file URL path).

Bundle.main.bundlePath
Bundle(for: Framework1.self).bundlePath
Bundle(for: Framework2.self).bundlePath

The URL will look like this:

~/Library/Developer/XCPGDevices/x-x-x/data/Containers/Bundle/Application/y-y-y/UIPlayground-28059-1.app
~/Library/Developer/Xcode/DerivedData/App-xyz/Build/Products/Debug-iphonesimulator/Framework1.framework
~/Library/Developer/Xcode/DerivedData/App-xyz/Build/Products/Debug-iphonesimulator/Framework2.framework

If you add your resources to the “Resources” group in Playground, then the main bundle will contain the resources.

But if you need a resource from a framework, then you need the bundle for the framework.

The Complicated Case

In our previous tutorial that uses Playground to create application’s UI, the scenario is complicated because the same resource is used in 2 targets:

1. The app
2. The framework for playground (MyUIPlaygroundFramework)

For such case, the resource has to specify the bundle correctly.

The trick is to make use of a class that exists in both the app and framework.

let bundle = Bundle(in: CommonClass.self)

When you use in app, bundle will be main bundle.

When you use in framework, bundle will be the framework’s bundle.